This issue with synthetics in this engine will be with them cleaning off old dried up seals that are now covered in varnish/deposits. Once this happens, the seals are expected to perform, well, like seals, and with them now being dryed-up and non-operative, you can expect some degree of either leaking or burning...maybe both.
I STRONGLY FEEL that your best bet, at least at this point until late fall, is to run a HDEO 15w-40 such as Delvac, Delo, or Pennzoil Long-Life.
You state you are experiencing blow-by, and the Long-Life does have the good dose of molybdenum in it (that significantly helps in regards to blowby), so I'll have to recommend that.
Running Long-Life from now until then will get the engines lubrication system back to running/operating like it should, and then, when the colder weather sets in, you can switch over to a 5w-40 of some sort.
So, I recommend Pennzoil Long-Life 15w-40 and a nice high-quality oil filter.